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efaq: Using regular expressions

 
 5.36 How do I use regexps (regular expressions) in Emacs?
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 (emacs)Regexp Backslash.
 
    The ‘or’ operator is ‘\|’, not ‘|’, and the grouping operators are
 ‘\(’ and ‘\)’.  Also, the string syntax for a backslash is ‘\\’.  To
 specify a regular expression like ‘xxx\(foo\|bar\)’ in a Lisp string,
 use ‘xxx\\(foo\\|bar\\)’.
 
    Note the doubled backslashes!
 
    • Unlike in Unix ‘grep’, ‘sed’, etc., a complement character set
      (‘[^...]’) can match a newline character (<LFD> a.k.a. ‘C-j’ a.k.a.
      ‘\n’), unless newline is mentioned as one of the characters not to
      match.
 
    • The character syntax regexps (e.g., ‘\sw’) are not meaningful
      inside character set regexps (e.g., ‘[aeiou]’).  (This is actually
      typical for regexp syntax.)
 

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